I. Field of the Invention
The present invention relates generally to frequency tracking loops used in communication systems, and more particularly to a frequency tracking loop for use in a communication system employing orthogonal Walsh modulation.
II. Description of the Related Art
In every communication system employing modulated communication signals, some mechanism must be provided to demodulate received signals. Further, in order to ensure accurate demodulation, the demodulator must be able to compensate for variations or shifts in the carrier frequency of the received signals.
One conventional technique used to implement such carrier shift tracking in a communication system demodulator is to use a Phase Locked Loop ("PLL"). This type of demodulator works well when the transmitted signal is modulated using conventional modulation techniques, and can be reasonably optimized for either a wide frequency range response or high accuracy. Where more sophisticated modulation schemes are used, however, certain properties of these modulation schemes can make traditional techniques much less useful or responsive.
This is particularly true in digital spread spectrum type communication systems employing M-ary orthogonal Walsh code modulation. As part of such techniques, groups of data symbols to be transferred are mapped into Walsh functions or codes that are transmitted. The received signal is demodulated with respect to a set of such Walsh codes to establish a likelihood as to which codes were transmitted to establish what data symbols are being transferred. However, frequency errors in tracking such signals, especially in the presence of noise, greatly reduces the ability to distinguish which Walsh code was received, and it can quickly become problematic for conventional tracking techniques to maintain frequency tracking.
Therefore, what is needed is a frequency tracking loop designed to take advantage of certain properties associated with M-ary orthogonal Walsh modulation to provide improved tracking abilities. Such an apparatus and tracking method would be useful in implementing more effective communication signal demodulators.